In recent years several processes for coating metal surfaces have been described which comprise contacting the metal surface with an acidic aqueous resin dispersion which is such that the acid etches the metal surface to generate metal ions and as a result the dispersion becomes destabilised adjacent the metal surface to deposit thereover a substantially uniform coating comprising the resin, and probably also the metal ions. The process has the particular advantage that it will give substantially uniform thickness irrespective of the shape of the article, whereas conventional techniques of applying a coating, such as brushing, give variable thickness. Many of these so-called "auto-phoresis" processes are listed in, for example, British Patent specification No. 1,431,843.
In practice the process generally requires the presence both of an acid and an oxidizing agent and most of the processes that have been described use hydrofluoric acid as the acid and hydrogen peroxide as the oxidizing agent. It has of course long been recognized that in theory it might be possible to use other acids and other oxidizing agents. Thus in British Pat. specification No. 1,349,827 it is proposed to use, for example, sulphuric acid, hydrochloric acid, hydrofluoric acid, nitric acid, acetic acid and various others, but hydrofluoric acid is said to be preferred and is used in the Examples. Similarly again in the British Patent specification No. 1,349,827, various oxidizing agents are proposed for example, perborate, bromate, permanganate, nitrite, nitrate and chlorate but again hydrogen peroxide is preferred and is used in the Examples. In British Patent specification No. 1,241,991, phosphoric acid is said to be the preferred acid. A wide range of oxidizing agents are proposed, including benzoquinone, nitrophenol, persulfates and nitrates but again hydrogen peroxide is preferred.
Hydrofluoric acid is an undesirable material to use, especially in the large amounts, (e.g. up to 5% of the treatment solution) generally used in auto-phoresis processes. Hydrogen peroxide is a convenient oxidant in that it does not create any pollution problems but it tends to be unstable and so it is impossible to make up the treatment solution a long time before use. Instead, either it must be made up specially for use and/or peroxide must be added during the process. It has therefore been our object to devise a process that does not require hydrofluoric acid and hydrogen peroxide.